Devices and techniques for applying hot melt adhesives are well known in the art. Typically a thermoplastic adhesive in a solidified form is used such as made of polyolefins, copolymers, polyesters or polyamides. These adhesives are applied in a rod form to the inlet of a dispenser having a heated melting chamber in which the adhesive is melted and then dispensed from an outlet onto a surface where the solidifying material serves as an adhesive. See for example. U.S. Pat. No. 4,032,046 to Elliott et al.
This patent is typical of the techniques used to dispense hot melt adhesive in that a hand-holdable dispenser gun is provided into which a rod of thermoplastic adhesive is fed by wheels which together with a drive motor present a bulky and heavy structure that is mounted on the dispenser. A mechanism is provided to sufficiently retract the rod from the dispenser to create a void that absorbs the expansion of melted adhesive as well as avoids excessive pressurization of the chamber leading to leakage during idling. A ball valve is located in the outlet to prevent leakage. Similar dispensers are shown and described in U.S. Pat. Nos. 3,285,475 to Phillips; 3,612,357 to Ruskin which describes a manually activated rod drive; 4,090,643 to Wilkinson et al wherein a preformed adhesive rod is advanced by way of a pneumatically activated piston.
The retraction mechanism adds complexity and with the gun mounted motor drive creates a bulky dispenser that can easily become tiresome to use. The partial retraction of the rod from the melting chamber tends to cause fouling with melted and then solidified adhesive, tending to clog the dispenser which then requires expensive and time consuming overhaul. Frequently, melted adhesive flows back along space between the rod and the dispenser and then solidifies to make it very hard to feed the rod, particularly when a manual advancing force is used. In case of a motor drive for the rod the increased resistance from the solidified "melt-back" tends to cause excessive compression of the rod which deforms, thus increasing melt-back and subsequent fouling. Another common problem encountered is caused by temperatures at the inlet to the melting chamber that are above the softening temperature of the rod. Rod softening makes it more difficult to push the rod into the melting chamber, particularly when excessive resistance is encountered due to melt-back.
Techniques have been proposed to inhibit back flow of melted adhesive that tends to solidify between the rod and the dispenser and also maintain the strength of the rod. In U.S. Pat. No. 3,285,475 forced air cooling is applied to cooling discs mounted on a barrel through which the rod is fed to be melted. The cooling air is generally applied to the entry part of the barrel so that the transition zone of the rod from solid to a melted state is likely to be elongated. In such technique, the force required to push the rod into the melting chamber still tends to increase with usage as melted adhesive gradually creeps back towards the inlet, particularly each time after the dispenser is turned off.
Another technique to prevent leakage of molten adhesive back into the dispenser is described in U.S. Pat. No. 4,314,655 to Leibhard et al. In this patent a silicone gasket surrounds the rod prior to the inlet to a melting chamber with an insulator ring placed between the gasket and the inlet. This also would seem to eventually develop an elongate section of solidified melt-back between the rod of adhesive and the inlet leading to the melting chamber particularly each time after the rod drive is turned off.
The adhesive materials, even when they are melted, are quite viscous with a consistency of thick syrup, and thus require a force to extrude melted adhesive from the melting chamber. In typical dispensers this force is provided from a displacement of the melted adhesive by the rod. The rod drive must therefore, overcome the viscosity resistances of the melted adhesive as well as any friction encountered from a solidified melt-back. Since the melted adhesive expands, provisions are commonly also made to retract the rod form the melt chamber when melted adhesive is no longer needed lest excessive dribbling of melted adhesive occurs at the outlet. Rod retraction, however, tends to encourage the formation of solidified melt-back leading to a need to clean and interrupt usage of a dispenser.
Proposals have been made to avoid drip by use of a valve at the outlet of a melting chamber. See, for example, U.S. Pat. Nos. 3,653,552 to Ash; 3,285,475; 3,485,417 to Cocks; and 4,493,972 to Steinel et al. In such devices the melting chamber pressures must still be relieved by either allowing retraction of the rod or permitting a drip in case of excessive pressure lest expansion of the melted adhesive would burst the dispenser.
In some hot-melt dispensers the rod is manually advanced by activation of a trigger. This becomes tiresome to an operator when high resistance is encountered. Substitution of motor activated driven wheels makes the dispenser more bulky and thus more difficult to manipulate and requires good frictional gripping of the rod to push it into the melting chamber. When a lubricant is used on the rod to facilitate its entry into the melting chamber, a frictional drive is made more difficult because the amount of wheel pressure on a rod cannot be made so high lest it causes deformation, which would permit more melt back leading to more friction and then requiring more drive force tending to cause further rod deformation, etc.
When a foamed adhesive is to be dispensed, very high pressures are encountered in the dispenser; see, for example, U.S. Pat. Nos. 4,059,466 and 4,059,714 to Scholl et al and 4,396,529 to Price et al. In such foamed adhesive dispensers, melted adhesive is pumped from a reservoir to a mixing chamber where the molten material is mixed with pressurized air and then discharged as a foam. Such dispenser does not lend itself to a manually held dispenser operation, is difficult to keep clean and requires that all of its operating parts in contact with the material are at a high temperature.
The foaming of thermoplastic materials is well known in the art. In U.S. Pat. Nos. 3,470,113 to Baxman et al. and 4,163,037 to Niznik for example, techniques are described to produce foamable substances using a blowing agent such as Celogen and a polyolefine. The substances are then foamed in a subsequent molding process during which the temperature is sufficiently high to activate the blowing agent. In an article entitled "Crosslinked PE Foam Sheet: New Continuous Process Arrives," published in PLASTICS TECHNOLOGY, of November, 1980 at pages 89-92, raw materials are blended and extruded to form an unblown sheet containing a blowing agent. The unblown sheet can be stored and foamed at a later time in an oven wherein the plastic sheet is passed over jets of hot air. The air crosslinks the material and initiates liberation of the blowing agent while preventing the now soft and tacky sheet from sticking to the conveyor belt. Temperatures are tightly controlled to prevent entrapment of gas in cross-linked closed foam cells.